Conventionally, decanter type centrifugal dehydration devices have been widely used for performing solid/liquid separation of sludge and the like (hereinafter, referred to as a processing liquid) by centrifugal force. Conventional decanter type centrifugal dehydration devices have disadvantages including an occurrence of a phenomenon where centrifugal force weakens and moisture content increases due to a shorter distance from a center of rotation (radius) to a cone section of a bowl. The present applicants have previously provided a straight-drum centrifugal dehydration device (Patent Document 1) as a solution to such disadvantages in the conventional decanter type centrifugal dehydration devices. With the straight-drum centrifugal dehydration device described above, an inner peripheral wall of a bowl forms a cylindrical shape extending along a rotating shaft of the bowl, a discharge path for discharging a sedimentary heavy component to the outside of the bowl is provided in one end wall of the bowl, an opening of the discharge path into the bowl is provided in a vicinity of an inner peripheral wall of the bowl, the discharge path constitutes a choke passage which limits discharge quantity, a deposit layer of a dewatered cake in a compacted state is formed in a vicinity of the opening of the discharge path, and the dewatered cake in the compacted state is directly discharged via the discharge path by centrifugal head pressure which acts on the dewatered cake in accordance with a thickness of the deposit layer formed by a discharge resistance of the choke passage and by a transport force of a screw conveyor. Accordingly, since only a portion subjected to maximum compaction among the dewatered cake deposit layer in the bowl is directly discharged, a moisture content of a dewatered cake can be lowered to a level unparalleled by conventional centrifugal dehydration devices.
On the other hand, in addition to structural/functional improvements to a dehydration device such as described above, separation/dewatering processes are performed by adding one of or both of a polymeric flocculant and an inorganic flocculant to a processing liquid to form a floc in order to further enhance a dehydration effect. Methods of adding such flocculants differ depending on types of dehydrators and types of processing liquids. As a specific method of adding a flocculant when performing solid/liquid separation by a decanter type centrifugal dehydrator, a general method involves feeding a polymeric flocculant together with sludge into a sludge feed chamber in an inner drum from a multiunit tube (for example, refer to Patent Documents 2 and 3). However, other methods have been proposed, including a so-called two-liquid method in which a flocculated floc created in advance by adding a polymeric flocculant to sludge and agitating the sludge and the polymeric flocculant in an external agitation tank and is fed to a sludge feed chamber in an inner drum and, at the same time, an inorganic flocculant is added from inside the inner drum to a sludge deposit which moves toward a small diameter side while being dewatered in a tapered section on an inner periphery of an outer drum (Patent Document 4), and a method in which solid/liquid separation is performed on sludge fed with an inorganic flocculant and a polymeric flocculant in a centrifugal dehydration device and the inorganic flocculant is once again injected into the separated sludge (Patent Document 5).
With respect to the decanter type centrifugal dehydration devices mentioned above, Patent Document 3 describes that the method according to Patent Document 3 is capable of lowering a moisture content of dewatered sludge down to 82%. In addition, Patent Document 4 describes that the method according to Patent Document 4 is capable of lowering a dewatered cake moisture content of sewage-digested sludge to 72.7% using a table-top centrifugal dehydrator in a laboratory, and Patent Document 5 describes performing the method according to Patent Document 5 on similar sewage-digested sludge (sludge concentration 1.5%) at a processing quantity of 1.5 m3/h and a centrifugal effect of 2500 G resulted in a dewatered cake moisture content of 75.0%.
As described above, with conventional small-size decanter type centrifugal dehydrators, dewatered cake moisture content can only be lowered to around 75% at the most using a real machine in the case of digested sludge.